Search for the plugin of the other plasmoid

Summary:
Some applets use completely the code of another one with the key
X-Plasma-RootPath.
At the moment it doesn't work on applets that have a binary plugin portion
This makes pluginloader load the plugin of the original applet we're using

Test Plan:
an applet that falls back to one that has a binary plugin now works.
ones that fall back to applets without binary plugin still work
normal applets as well

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22675